In the manufacture of plastic containers, such as monolayer or multilayer PET containers, it is conventional to mold a container preform having a body and a finish with one or more external threads. The finish is typically molded to its final geometry, while the body of the preform is subsequently blow molded to the desired geometry of the container body. Although this manufacturing technique is satisfactory for fabrication of containers of differing finish diameters, the throughput of the process is greatly reduced when employed for fabricating preforms and containers of increased finish diameter. For example, a preform mold cavity block having ninety-six mold cavities for preforms with a 28 mm finish diameter would typically accommodate only forty-eight cavities having a 43 mm finish diameter for the same overall cavity block size. This throughput is even further reduced for wide-mouth preforms having a finish diameter greater than about 2.0 inches or 50 mm.
To address this manufacturing throughput problem, it has been proposed to fabricate a wide-mouth container by molding a narrow-neck preform, and then blow molding the preform body within a cavity that forms the container body, the container finish with threads, and a trim moil or dome that must be removed from the container body and finish, along with the preform finish, after the container is removed from the mold. Although this technique permits use of narrow-neck preforms, and thus maintains high throughput during the preform molding stage, the technique has the disadvantage that the moil and preform finish constitute trim scrap that must be recycled or discarded. Furthermore, when fabricating multilayer contains having an intermediate layer of barrier resin for example, this technique presents the disadvantage that an edge of the barrier layer is exposed at the moil trim plane, which can result in substantial water vapor absorption and loss of barrier properties for many hydrophilic barrier resins such as ethylene vinyl alcohol (EVOH). Moreover, and perhaps most importantly, the external threads on the container finish are blow molded in this technique, and are not as sharply defined and detailed as are external threads formed by direct molding.
It is therefore a general object of the present invention to provide a method of making a plastic container, and/or a plastic container formed by such method, in which the container threads are formed by pressure molding (compression or injection molding) rather than blow molding, and which maintains high throughput in the preform molding stage.
A method of making a plastic container having a body and a finish with at least one external thread, in accordance with a first aspect of the present invention, includes pressure molding (i.e., injection or compression molding) a plastic preform having a body and a finish with at least one external thread, blow molding the body of the preform to form the body of the container, and either prior to, subsequent to or both prior and subsequent to blow molding the container body, expanding the pressure molded preform finish to form a container finish having at least one pressure molded external thread. In the preferred embodiments of the invention, the finish is expanded after the container body is blow molded. The invention can be employed to expand the preform or container finish from a small narrow-neck diameter such as 28 mm to a larger narrow-neck diameter such as 43 mm. The invention can also be implemented to expand a narrow-neck preform or container finish to a wide-mouth preform or container finish, such as 28 mm to 63 mm or 48 mm to 63 mm. The invention can also be implemented to expand a wide-mouth finish into a larger wide-mouth finish, such as 63 mm to 83 mm.